Insulated electrical apparatus



Sept. 6, 1966 D. BERG 3,271,711

INSULATED ELECTRICAL APPARATUS Original Filed Feb. 25, 1963 '3 E :40 c 5(C3 F7)2 SF4 o 4 F9SF5 5 3 u o J (c F) SF 2 4 9 2 4 20 8 0 4 w o AIR n O0 l l I I l 0 I0 20 30 40 5o INVENTOR.

ABSOLUTE PRESSURE IN CM OF HG.

Dome! Berg Fl g2.

ATTORNEY United States Patent 3,271,711 INSULATED ELECTRICAL APPARATUSDaniel Berg, Churchill Boro, Pa., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Continuationof application Ser. No. 260,794, Feb. 25,

1963. This application Mar. 2, 1965, Ser. No. 438,466 3 Claims. (Cl.336-57) This application is a continuation of application Serial No.260,794, dated February 25, 1963, now abandoned.

This invention relates to an electrical apparatus which includes a fluidmeans for cooling and insulating the apparatus. This invention alsorelates to a process for cooling and insulating an electrical apparatus.

It is well-known in the art to insulate an enclosed electrical apparatuswith a liquid dielectric by immersing the apparatus in the liquid. Theliquid functions to both cool and electrically insulate the operatingelectrical elements during use. There are certain disadvantages inherentin the use of a liquid dielectric for this purpose. One disadvantage isthe necessity for employing large quantities of the liquid dielectric.Moreover, these liquid dielectrics require frequent attention andmaintenance to prevent deterioration. Furthermore, there is the dangerthat if an electric arc develops within such a liquid dielectric, anexplosion or fire may result, if the liquid dielectric is inflammable,or noxious and corrosive fumes may be emitted if the liquid dielectricis a halogenated compound.

Enclosed electrical devices which are insulated with vairous gaseousdielectric materials are also known in the art. However, it is generallynecessary to use known gaseous dielectrics under considerable pressurein order to improve their dielectric breakdown strength. The use ofthese high pressures is frequently objectionable since exceptionallystrong casings enclosing the electrical apparatus must be employed andgreat care must be exercised in gasketing and sealing of the joints inthe casings. Any leak in the casing is usually fatal to the satisfactoryoperation of such an apparatus.

It is also known in the art to cool an electrical apparatus by flowingover the apparatus a thin layer or film of a liquid material, such as aliquid fiuorinated organic compound. The compounds used are ones which,in the vapor :phase, are dielectrics. When such a compound in the liquidstate is passed over an electrical apparatus while in use, the apparatusis cooled by evaporation of the liquid and the resultant vapors of thecompound provide insulation for the electrical winding. One of thephysical properties essential to the widespread use of such a compoundis high electric strength of the vapors. It is highly desirable thatsuch a compound in the vapor state have a high electric strength at apressure low enough so that the casing which encloses the electricalapparatus need not have particularly heavy walls to resist the pressure,and that the compound has a relatively low boiling point so that it willbe converted to vapor at low temperatures in order that low cost solidinsulation may be used.

It is an object of this invention to 0001 an electrical apparatus byflowing over the apparatus a liquid which effects cooling byevaporation, the evolved vapors of the liquid providing insulation forthe apparatus.

It is another object of this invention to provide a method for coolingan electrical apparatus by contacting the apparatus with a liquidcompound which, upon contact with the hot portions of the apparatus,vaporizes, thereby cooling the apparatus by evaporation, and the Vaporsserve to insulate the apparatus, the liquid being one which is convertedto vapor at relatively low temperatures and the vapors having a highdielectric strength at relatively low pressures.

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Briefly, the objects of this invention are obtained by providing anelectrical apparatus, such as a transformer, comprised of a casingenclosing a source of heat. Thus, there may be disposed within thecasing conductor members which are adapted to have an electricalpotential developed across them and between which electrical dischargesmay occur, such as, for example, an electrical winding and a core. Suchan electrical apparatus is one which, in use, develops heat. Providedwithin the casing is a supply of a liquid perfluoroalkyl derivative ofSP Specifically, the liquid may be C F SF (C F SF or (C3F7)2SF4. Duringuse of the electrical apparatus, the liquid is distributed over theelectrical apparatus, for example, over the core and electrical winding.The apparatus is thereby cooled by evaporation of the liquid. Theevolved vapors from the evaporating liquid provide an insulating gaseousatmosphere within the casing.

The compound C.,F SF which may be used in the practice of thisinvention, has a molecular weight of 346, a boiling point of 705 C., arefractive index at 25 C. of 1.2701, a density at 25 C. of 1.839 and adielectric constant of 2.068. This compound has a high vapor pressure.Because of the unique combination of physical properties of C.,F SF itmay be used as a coolant and insulating medium in an electricalapparatus at lower operating temperatures and pressures than compoundsheretofore known such, for example, as SP itself.

The compound (C.,F SF which may also be used in practicing theinvention, has a molecular weight of 546, a boiling point of 154 C., arefractive index at 25 C. of 1.2943, a density at 25 C. of 1.903, and adielectric constant of 1.951. It requires a higher operating temperaturethan C.,F SF to attain the same pressure.

The compound (C F SF has a molecular weight of 446, a boiling point of116 C., a refractive index at 25 C. of 1.2856, a density at 25 C. of1.865, and a dielectric constant of 1.923.

These three perfluoroalkyl derivatives of SP have been found to giveoutstanding results in the vapor cooling of electrical apparatus.

The invention will be more fully described with reference to theaccompanying drawings wherein:

FIGURE 1 is a view, partly in section, through a transformer constructedaccording to the practice of this invention; and

FIG. 2 is a plot of breakdown voltages for the novel vapor-coolingcompounds and for air as a function of pressure.

The drawings will be more fully explained hereinafter.

In FIGURE 1, there is shown a transformer 10 having a core 11 andassociated electrical windings 12 disposed within a sealed, gas-tightcasing 13. The casing 13 may contain an insulating material 13a in orderto maintain a near constant temperature and pressure within thetransformer.

The core 11 is mounted on a grounded supporting member 14. Solidinsulating barriers 15 are interposed between the windings 12 and thesupporting member 14.

The windings 12 are provided with leads 16 which are connected to thebushings 17.

A sump 18 is situated in the lower .portion of the casing 13. The sump18 contains a supply of one of the novel liquids 19. The supply of thisliquid is relatively small with respect to the total size of the casing13. The liquid 19 while in the sump 18 is preferably maintained out ofcontact with the core 11 and the electrical windings 12. The liquid 19is present in suificient quantity to completely fill the casing 13 withvapor during the operation of the apparatus and still maintain a reserveof liquid.

A pump 20 is provided to circulate the liquid 19 through a conduit 21 toa spray device 22. The pump 20 is connected to the bottom of the sump 18by means of the pipe 23. When the transformer is in operation, theliquid 19 which is delivered to the spray device 22 by means of the pump20, is distributed over the core 11 and the electrical windings 12. Theliquid when thus delivered distributes itself as a thin film over thecore and the electrical winding and is evaporated, thereby cooling thecore and the electrical windings and forming an insulating gaseousatmosphere within the casing. The cooling thus produced has been foundto be very efficient and effective in maintaining uniform temperaturesof the electrical elements. Evolved vapors of the compound which fillthe transformer eventually flow to the condensing chamber 24 through theinlet 25 where the vapors condense. The condensed liquid then runs backinto the transformer and to the sump 18 through the outlet 2-6. In somecases, it will not be necessary to use a condensing chamber if the wallsof the casing 13 have sufiicient area to dissipate the developed heat tothe atmosphere. FIGURE 2 is a plot of the breakdown voltages for air andthe vapors of the novel coolant liquids as a function of pressure. Indetermining the values plotted in FIG- URE 2, the cell used was one inwhich the electrode comprised two l-inch steel spheres spaced 0.6 cm.apart. Only A.C. voltages were used. In determining the values for thevapors of the coolant liquids, the cell was filled with about cc. ofliquid and then was placed in an oil bath and the temperature varied toobtain the desired vapor pressure. The pressure was measured with amercury manometer attached to the cell. The results for air wereobtained by filling the cell to various pressures with air. It isevident from FIGURE 2 that the vapors of the compounds have extremelyhigh breakdown strengths as compared to air. Since they possess highelectric strengths at relatively low pressures, there is no need for anexceptionally strong casing enclosing the electrical apparatus.Moreover, in view of the relatively low boiling points of the compounds,the operating temperatures of the electrical apparatus is much lowerthan is usually the case. Therefore, low cost materials such aspressboard and paper may be used as the solid insulation in theapparatus such as, for example, the insulation 13a in the casing wallsand the material in the insulating barriers shown in FIGURE 1.

The advantages of this invention are not limited to the transformerwhich is illustrated but may be applied to other types of electricaldevices such as for example switches, X-ray tube housings, capacitors,generators, ca-

. bles, reactors, etc.

have electrical potentials developed thereaoross and be tween whichelectrical discharges may occur, said electrical device developing heatduring the use thereof; means for cooling said electrical devicecomprising a liquid selected from the group consisting of C F SF (C F SFand (C3Fq)2SF4, and means for distributing said liquid over saidelectrical device when in use, whereby cooling of said electrical deviceis effected by evaporation of said liquid, the vapors of said liquidprov-iding for electrically insulating said conductor members.

of said electrical device.

3. An electrical transformer comprising a sealed casing, an electricalwinding and core in said casing, a supply of a liquid selected from thegroup consisting of C4F9SF5, (C4F9)2SF4, and (C3F7)2SF4fdiS\POSed saidcasing, and means for distributing said liquid over said core andelectrical winding, said liquid serving to cool said core and electricalwinding by evaporation, the evolved vapors providing an insulatinggaseous atmosphere w-ithin said casing.

References Cited by the Examiner UNITED STATES PATENTS 2,561,738 7/1951Hill 336-57 OTHER REFERENCES Hoffman et al., Journal Of AmericanChemical Society, vol. 79, pp. 3424-3429, July 5, 1957.

LEWIS H. MYERS, Primary Examiner.

ROBERT K. SCHAEFER, Examiner.

T. J. KO'ZMA, Assistant Examiner.

4/1962 Camilli '3-3657

1. AN ELECTRICAL APPARATUS COMPRISING AN ENCLOSED CASING, A SOURCE OFHEAT DISPOSED WITHIN SAID CASING, A SUPPLY OF LIQUID FOR DISSIPATING THEHEAT FROM SAID SOURCE OF HEAT, SAID LIQUID BEING SELECTED FROM THE GROUPCONSISTING OF C4F9SF5, (C4F9)2SF4, AND (C3F7)2SF4, AND MEANS FORDISTRIBUTING SAID LIQUID OVER SAID SOURCE OF HEAT TO COOL IT BYEVAPORATING OF SAID LIQUID.